When preparing to start a generator to supply electrical power to a grid or other network, it is known to run the electrical machine constituting the generator as a motor in order to turn the prime mover and run it up to speed. To control the machine in this motoring mode the electrical machine is controlled by an inverter. The machine consumes electrical power from the grid and drives the prime mover so that it can be started and run up to speed. After operating speed is reached the machine can be configured to run as a generator driven by the prime mover and can feed electrical power to the grid. Examples of prime movers are different types of turbines and internal combustion engines but are not limited to these.
Typically, an asynchronous electrical machine, such as a conventional squirrel cage machine, is connected to a grid to supply active power.
The main grid network of a country or region is dominated by major sources of electrical power. For example, as a consequence of the decision by the government of Germany to move away from the generation of electrical power using nuclear sources, the grid is no longer dominated by a small number of big power plants but by an increasing number of smaller contributers. Besides the contribution of active power these contributers need to take part in the contribution of reactive power as well, as the major sources of electrical power have done up to now.
This presents a significant problem for the suppliers of electrical power, with systems being based on asynchronous machines that provide active power only. The cost of retro-fitting electrical generator sets with a reactive power outputs capability would be prohibitive. A cost-effective solution is needed that enables an active power generator plant to be able to be converted to delivering reactive power, for example, in compliance with legislation as set out above. The disclosed embodiments herein are applicable to the above situation or to any situation in which reactive power is required.